Xander was way more important to and for Willow than Willow was to him.
Emotionally Xander was both Willow's best friend and her big secret crush as they grew up. Xander was Willow's validation and bolstered her self worth. Willow was his best friend (along with Jesse?).
Once Buffy arrived all the dynamics changed. Both Xander and Willow focused on Buffy and the mission, and both drew confidence and validation from that. However, at her core Willow still needed Xander's unwavering love. Even after she had Oz she fell for Xander's attention.
Willow knew that Xander was the one person who would always love her no matter what. Sometimes she would be callous - Xander still loved her. Sometimes strange - still Xander loved. Sometime she'd flay people alive and try to destroy the world - and still Xander would not quit on her.
After Xander's very brief tryst with Willow, he drew far more from his friendship with Buffy to define his self worth, and his relationship to Anya for his growth and intimacy.
Oh wait - I said "for and to" didn't I? That's wrong.
Xander was much more important for Willow than vice versa. She needed him in ways that he didn't need her.
Willow was also more important to Xander than Xander was to her. She valued her relationships with Oz, Tara and Buffy more, I think. And her loyalties tended to show that. I think Xander would've sacrificed Anya for Willow. He demonstrated his willingness to sacrifice himself for Willow. I think only Buffy was more important to Xander than Willow and that's because he recognized the rightness of her mission. (Though he consistently challenged her vision of the mission when he thought she was wrong.)
In short, Willow took Xander for granted even though she depended on him. And Xander valued Willow highly long after he got much of anything in return from her.